Determining the Two-Photon Absorption Cross-Section for the 5 2 S 1/2 --> 5 2 D 5/2 Transition in Naturally Occurring Rubidium

Recently we demonstrated lasing at 420 nm and 455 nm inside of rubidium and cesium vapor cells respectively, by non-resonant, two photon absorption at near infrared pump wavelengths. Two photon absorption from the ground S1/2 state to excited D3/2,5/2 or higher lying S1/2 state via virtual states and subsequent infrared stimulated emission pumps the upper laser levels, the second excited P1/2,3/2 states. To better understand the mechanism and phenomenology of this type of laser, this paper presents a method for determining the two-photon absorption (TPA) cross-section for the 5S1/2 −→ 5D5/2 transition in naturally occurring rubidium. For an incident pump intensity of 392 W cm2 and a rubidium concentration of 3.34 × 10 atoms cm3 , a peak absorbance of 0.10 is observed. The concentration of the Rb in the test cell is determined by one photon absorption spectra of the D2, indicating a vapor curve shifted by 14 C from the exterior wall temperature. The incident laser beam is focused to a waist of 75 μm. The beam quality is consistent with an M = 1.05 − 1.09. A model for the intensity as a function of propagation distance is developed to interpret the observed absorption spectrum and determine a cross-section for the two photon absorption.